74 PAST HISTORY OF ANIMALS. 



tion of any given animal are very great. Many are destroyed 

 by other living creatures, or obliterated by chemical agencies. 

 Except in rare instances, only hard parts, such as bones, 

 teeth, and shells, are likely to be preserved, and this at once 

 greatly limits the evidential value of fossils. The primitive 

 forms of life would almost certainly be without hard parts, 

 and have left no trace behind them. A number of ex- 

 tremely interesting forms, such as many worms and the 

 Ascidians, are, for the same reason, almost unrepresented 

 in the rocks. Finally, we cannot suppose that such an 

 external structure as a shell can always be an exact index of 

 the animal within. Some shells, such as Nautilus and some 

 of the Brachiopods, occur as fossils from remote Palaeozoic 

 ages onward, but it is impossible to believe that the animal 

 within has never varied during this period, though we can- 

 not now learn either the nature or the amount of the 

 variation. 



After fossilisation has taken place, the rock with its con- 

 tents may be entirely destroyed by subsequent denudation, 

 or so altered by metamorphic changes that all trace of 

 organic life disappears. Of those fossils which have been 

 preserved only a small percentage are available, for vast 

 areas of fossiliferous rocks are covered over by later deposits, 

 or now lie below the sea or in areas which have not yet 

 been explored. 



With all these causes operating against the likelihood of 

 preservation, and of finding those forms that may have been 

 preserved, it is little wonder if the geological record is 

 incomplete ; but such as it is, it is in general agreement 

 with what the other evidence, theoretical and actual, leads 

 us to expect as to the relative age of the great types of 

 animal life. Further, those specially favourable cases which 

 have been completely worked out have yielded results 

 which strongly support the general theory. 



Probabilities of "fossils." — But it will be useful to note the 

 probabilities of a good representation of extinct forms in the various 

 classes of animals. Thus among the Protozoa the Infusoria have no 

 very hard parts, and have therefore almost no chance of preservation, 

 and the same may be said of forms like Amoebae ; while the Foramini- 

 fera and the Radiolaria, having hard structures of lime or silica, have 

 been well preserved. The Sponges are well represented by their spicules 

 and skeletons. Of the Coelentera, except an extinct order known as 



